Jackie Robinson undervalued compared to Mantle?

[Tails01]

Why don't Jackie Robinson Topps cards command more value relative to Mantle?

Jackie appears in Topps sets from 1952 to 1956 (5 years).

Mantle appears in Topps sets from 1952 to 1969 (16 years because no 1954 or 1955 because of an exclusive contract with Bowman).

Slabbed Mantle cards typically double the same grade as Jackie cards (take 1956, for example), and yet there are more than three times the Mantle cards in existence.

Was Mantle really that much better?

Peak 5-year WAR goes to Mickey, but career WAR is very close (110 vs. 107).